Design is a traditional task common to all engineering fields although most engineers do not have to deal with the heterogeneous properties of materials as is the case in site remediation, mining, geotechnical engineering, etc. A structural engineer can specify properties of steel or reinforced concrete and an electronics engineer can specify the physical properties of transistors, capacitors, etc. but an engineer designing a remediation system cannot so specify material properties. It falls to someone I refer to as the contaminant hydrogeologist to specify the properties of the soil and/or rock and the spatial distribution of contaminants at a site so that the design engineer can create a remediation programme that will meet the required cleanup concentrations in soil, rock and groundwater at an acceptable cost. I present a number of examples that illustrate how critical it is for the specification of the physical and chemical properties of a contaminated site to be done correctly and how necessary it is to have an appropriate geological model for that purpose. These examples include chlorinated degreasing solvents at a site in Utah, perchlorate in northern California and gasoline in southern California.